Water-cooled nuclear reactors, especially reactors cooled by pressurized water, have a core consisting of prism-shaped fuel assemblies which are placed next to one another and arranged vertically and which rest on the lower core plate of the nuclear reactor by means of a lower connector. The cooling water of the reactor passes through the lower connector commonly called "bottom nozzle" and circulates from the bottom upwards in the vertical direction within the assembly.
This cooling water is liable to contain particles which it carries into the assembly through orifices passing through the adaptor plate of the lower connector. The particles come to rest in gaps located between the fuel rods and the walls of the cells of the first spacer grid of the assembly, i.e. the spacer grid located nearest to the lower connector of the assembly.
The circulating fluid subjects the particles to axial and transverse stresses, to the extent that this can result in wear of the jacket of the fuel rod. It is therefore desirable to retain the particles contained in the cooling fluid, in order to prevent them from penetrating into the assembly.
U.S. Pat. No. 4,664,880 and U.S. Pat. No. 4,678,627, for example, disclosed retention devices in the form of structures made of metal wires or of pierced sheet metal and arranged in the lower connector of the assembly.
It has also been proposed, in U.S. Pat. No. 4,684,496, to use as a retention device a filtration plate produced in the form of a grating consisting of wafers and associated with the lower connector of the assembly.
These known devices generally introduce a considerable head loss into the circulation of the cooling fluid of the reactor, inasmuch as the fluid passage cross-sections projected in a plane perpendicular to the flow cross-section may only be small.
EP-A-0,289,829 and EP-A-0,311,037 have also proposed producing the filtration device by using the very adaptor plate of the lower connector as a filter element, either by providing a special duct in this plate or by inserting filtration means locally in orifices machined specially in the adaptor plate. However, special machining of this essential structural element of the lower connector has disadvantages.